In general, an injection unit in an injection molding machine includes a front plate 1, a moving plate 2 and a rear plate 3, in this order from the front, opposite to a clamping unit (not shown), as illustrated in FIG. 2. The front plate 1 includes a nose member 5 projecting from a plate body 4 toward the front. A hopper 6 and a heating cylinder 7 are attached to the nose member 5. Further, a resin guiding hole 9, which penetrates into the heating cylinder 7 from the hopper 6 and reaches to a screw 8, is formed in the nose member 5. In FIG. 2, reference numeral 10 denotes a tie bar linking the front plate 1 to the rear plate 3. The moving plate 2 in which the screw 8 is attached to its front is slidably fitted to the tie bar 10 in the front and rear direction. A ball screw 11 is pivotally attached to the rear plate 3, and its front portion is screwed into a ball nut 12 which is fixed to the rear side of the moving plate 2. A pulley 14, which is connected in association with a servo motor (not shown) for injection, is fixed to the backward projecting end of the rear plate 3 to which the ball screw 11 is attached. When the pulley 14 is rotated by the servo motor for injection, the ball screw 11 is rotated, and the ball nut 12 or the moving plate 2 is moved forward for injecting operation.
In order to make a resin flow smooth, it is necessary to grind the resin guiding hole 9, which is formed in the nose member 5 of the front plate 1, a constituent of an injection unit. Further, in order to prevent an inner wall of the resin guiding hole from being corroded due to effects of water and a corrosive gas caused by a resin, the inner wall portion may be provided with corrosion-resistant plating or a lining with a thin stainless tube fit into the hole. Conventionally, however, it has been customary to integrally form the front member 5 with the front body 4 of the front plate 1 as shown in FIG. 2, and thus it has been cumbersome and time-consuming to machine the resin guiding hole in the nose member 5, since such machining operation involves handling the whole front plate having a complex shape and large weight.
Further, the resin is melted with the rotational operation of the screw 8 in the heating cylinder 7, which is kept heated at a high temperature with a heater. On the other hand, the nose member 5 of the front plate 1 is cooled lest a bridge is formed in the resin guiding hole due to the heated resin supplied from the hopper 6, or preheating is required depending on the characteristic of resin. For this reason, it is necessary to suitably control the temperature of the nose member so that a predetermined temperature can be maintained. However, being integrally formed with the plate body 4 of the front plate 1, and having a small heat capacity, the nose member 5 is apt to be thermally influenced by the plate body 4, and so it has been difficult to control its temperature at a constant level either in cooling or heating. In addition, the conventional heating cylinder has been directly and closely attached to the nose member 5, and so it is apt to be affected by the temperature of the heating cylinder 7.